Add USB port mapping instructions to README.md

- Introduced a new section detailing the `gen_udev_rules.py` script for stable USB port mapping.
- Explained features, usage, and installation steps for generated udev rules to ensure consistent device identification across reboots.

README.md

@@ -128,6 +128,37 @@ python3 esp32_deploy.py \
 
 See `doc/DEPLOYMENT_GUIDE.md` for complete deployment instructions.
 
+### USB Port Mapping (`gen_udev_rules.py`)
+
+When deploying to multiple ESP32 devices, Linux assigns `/dev/ttyUSB*` ports that can change between reboots or when devices are reconnected. The `gen_udev_rules.py` script creates stable symbolic links (e.g., `/dev/esp_port_01`, `/dev/esp_port_02`) based on each device's USB physical topology (`ID_PATH`), ensuring consistent port identification across system reboots.
+
+**Features:**
+- Uses `ID_PATH` for stable device identification (not just port numbers)
+- Creates predictable symlinks: `/dev/esp_port_01`, `/dev/esp_port_02`, etc.
+- Supports full scan (generate from scratch) or incremental updates (append new devices)
+- Sorts devices by physical USB topology for consistent ordering
+
+**Usage:**
+
+```bash
+# Generate complete rules file from scratch
+python3 gen_udev_rules.py --full
+
+# Append only new devices to existing rules (default mode)
+python3 gen_udev_rules.py --append
+
+# Dry run to preview changes
+python3 gen_udev_rules.py --append --dry-run
+```
+
+After generating rules, install them:
+```bash
+sudo cp 99-esp32-stable.rules /etc/udev/rules.d/
+sudo udevadm control --reload-rules && sudo udevadm trigger
+```
+
+The deployment script (`esp32_deploy.py`) can use these stable symlinks when `--map-ports` is used, or devices can be referenced directly by their `esp_port_XX` names.
+
 ## Project Structure
 
 ```